Download A Comparison Between the Shear Strength Measured with

A Comparison Between the Shear Strength Measured with Direct Shear and Triaxial
Devices on Undisturbed and Remolded Soils
Une comparaison entre la résistance au cisaillement mesurée avec appareils de cisaillement direct
et triaxiaux sur les sols non remaniés et remoulés
Castellanos B.A., Brandon T.L.
Virginia Polytechnic Institute and State University
ABSTRACT: A comparison is presented between the shear strength measured with direct shear and triaxial devices based on the
results from tests conducted on undisturbed and remolded soils. A series of consolidated-undrained (CU) triaxial tests and
consolidated-drained (CD) direct shear tests were performed on undisturbed samples from the New Orleans area. These tests were
conducted on soils ranging from low plasticity silts to organic fat clays. The results from the undisturbed samples showed that the
drained friction angles obtained from the CU triaxial tests were considerably higher than those obtained from the CD direct shear
tests. On the other hand, results obtained from remolded test specimens originating from a variety of locations showed almost no
difference in the effective stress shear strength. The difference in the results can be explained in part by the fact that natural soils can
have a preferred particle orientation or anisotropic fabric based on the deposition or formation of the soil. In the case of the New
Orleans soils, the undisturbed samples exhibited evidence of horizontal deposition. In remolded samples, the soil is more much more
homogeneous and isotropic, so every plane in the soil may be expected to have similar shear strength.
RÉSUMÉ : En utilisant des résultats de tests effectués sur des sols non perturbés et remoulés, une comparaison entre la résistance au
cisaillement mesurée par cisaillement direct et avec des appareils triaxiaux est présentée. Une série d’essais triaxiaux consolidés non
drainés et d’essais de cisaillement direct consolidés drainés ont été effectués sur des sols non remaniés de la région de la NouvelleOrléans. Ces tests ont été effectués sur des sols allant de vases de plasticité faible à des argiles organiques de haute plasticité. Les
résultats des échantillons non perturbés ont démontré que les angles de friction drainés obtenus à partir d’essais triaxiaux consolidés
non drainés sont considérablement plus élevés que ceux obtenus à partir d’essais de cisaillement direct. D’autre part, les résultats
obtenus à partir de spécimens remoulés originaires de lieux divers n’ont montré pratiquement aucune différence dans la résistance
effective au cisaillement. La différence des résultats peut s’expliquer en partie par le fait que les sols naturels peuvent avoir une
orientation préférée des particules ou un tissu anisotrope basé sur le dépôt ou la formation du sol. Dans le cas des sols de la NouvelleOrléans, les échantillons intacts ont présenté des signes de déposition horizontale. Dans les échantillons remoulés le sol est beaucoup
plus homogène et isotrope, de sorte que l’on peut s’attendre à ce que chaque plan du sol ait la même résistance au cisaillement.
KEYWORDS: direct shear, triaxial, shear strength, fully softened, remolded, undisturbed, shear strength measurement.
1
INTRODUCTION
The triaxial and direct shear devices have been historically used
successfully to measure the peak shear strength of soils. In
geotechnical projects, these tests are often used interchangeably
to determine effective stress or drained shear strength
parameters without regard to the potential difference in the
results. A series of consolidated-undrained (CU) triaxial tests
and consolidated-drained (CD) direct shear tests were
performed on undisturbed samples from the New Orleans area.
These tests were conducted on soils ranging from low plasticity
silts to organic fat clays. The bulk of these tests were conducted
as part of the reconstruction of the flood protection system,
which was severely damaged by Hurricane Katrina in 2005.
The city of New Orleans is located on alluvial soils that are
part of the delta formed by the sediments of the Mississippi
River. According to Dunbar and Britsch (2008), the surficial
soils were formed during the Holocene and consists of fine
grained soils extending to about 17 m to 25 m deep in most of
the area, while exceeding 46 m deep at some locations. Dunbar
and Britsch (2008) stated that this layer is characterized by
stacked and generally horizontal layering created by the
deposition mechanism.
Numerous CU triaxial compression tests and CD direct shear
tests were performed on undisturbed test specimens trimmed
from 125 mm diameter tube samples to characterize the
effective stress shear strength parameters of the soils in the
greater New Orleans area. These test results were assessed to
317
allow a comparison of the effective stress shear strength
parameters for the undisturbed samples based on soil type,
plasticity characteristics, etc. The tests conducted on the
undisturbed tube samples were assessed based on the peak shear
strength.
A series of direct shear tests and triaxial tests were also
conducted on remolded soil specimens to determine the fully
softened shear strength. The fully softened shear strength was
defined by Skempton (1970) as the drained peak shear strength
of a clay in its normally consolidated state. According to
Skempton (1977), the fully softened shear strength can be
measured on remolded normally consolidated specimens. Three
different soils from various locations in the USA were tested to
examine the difference in the fully softened strength parameters
obtained from triaxial and direct shear test apparatuses.
2
PREVIOUS WORK
The direct shear and triaxial devices have been used for over 70
years to determine drained shear strength parameters of soils
(Saada and Townsend 1981). These two devices have marked
differences in the stress condition that is developed in the test
specimen. Some of the biggest differences are: 1) strain
boundary conditions, 2) failure plane orientations, and 3)
principal stress orientation. In the triaxial device, the
intermediate and minor principal stresses are equal and are
normally specified at the beginning of the test. In the direct
Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
318
these two devices will provide the same effective stress shear
strength parameters. Investigators that have found agreement in
the results did not state whether the soil tested had a preferred
particle orientation or layering that could influence the results.
3
DATA COLLECTION AND RESULTS
Undisturbed test specimens
3.1
A subset of 63 CU triaxial test series and 146 CD direct shear
test series was selected from the test results available from the
New Orleans investigation. Each test series normally consisted
of three individual tests conducted at different confining
pressures. Only high quality tests were selected. The test results
selected had to comply with the following requirements: 1) At
least two test specimens were consolidated to stresses that were
higher than the preconsolidation stress; 2) The end of primary
consolidation was achieved during the consolidation stage of the
test, 3) Index properties were available; 4) A peak deviator
stress was reached for CU triaxial tests in less than 15% strain
and a peak shear stress was reached for CD direct shear tests at
a horizontal displacement less than 0.4 inches.
Since all the samples used were normally consolidated, the
shear strength interpretation assumed that the effective stress
cohesion intercept was zero. The least-squares method was used
to obtain the corresponding effective stress friction angle.
Shown in Figure 1 are the values of the effective stress
friction angles as a function of the Plasticity Index (PI) for the
undisturbed New Orleans area test specimens for CD direct
shear and CU triaxial tests. The trend lines shown on this figure
are based on a statistical analysis performed by the authors.
These lines are presented to better show the trend of the data
and are not intended to be used as a correlation to predict
friction angles. From this figure, it can be seen that the effective
stress friction angle measured with the CU triaxial device is
generally higher than that measured with the direct shear device.
This difference was found to increase with increasing plasticity
index of the soil. In general, the difference in friction angle
ranged from about 2 degrees to 5 degrees.
40
Effective Stress Friction Angle (deg)
shear device, the magnitudes of the intermediate and minor
principal stresses are not known and are governed by the
vertical stress applied and the strength properties of the soil
being tested.
Progressive failure is the condition where the peak shear
strength is not mobilized in every point of the failure plane at
the same instance. This is caused by the non-uniform
distribution of strain in the failure plane combined with the
strain-softening characteristic of the soil. Some locations on the
failure plane will mobilize the peak shear strength while others,
having achieved more or less displacement, will mobilized a
shear strength below the peak shear strength. For the direct
shear device, investigators have reached different conclusions
about the effect of stress concentrations in this device. Hvorslev
(1960) measured horizontal displacements along the failure
plane in a direct shear device and found that the displacements
are not uniformly distributed, thereby causing progressive
failure to occur. Alternatively, a finite element study of the
direct shear box presented by Potts et al. (1987) showed that
although stress concentration exist on the failure plane of the
direct shear box, at the moment of failure the stresses on the
failure plane are more or less uniform and the peak shear
strength measured is not affected by progressive failure. For the
triaxial device, stress concentration caused by the end restraints
can influence the results. Research performed by Taylor (1941)
showed that if the ratio of length to diameter is between 1.5 and
2.5, the effect of the stress concentration is negligible.
In the direct shear device, the orientation of the principal
stresses on the failure plane varies during the shearing stage of
the test and the final orientation is unknown. In the triaxial test,
the major and minor principal stresses act on the horizontal and
vertical planes and this orientation does not change during
shear.
The orientation of the failure plane in the direct shear device
is predetermined as being near the midpoint between the upper
and lower halves of the shear box. In the triaxial device, the
orientation of the failure plane is governed by the soil structure
and the strength properties of the soil.
A literature review was undertaken to locate previous
comparisons of the shear strength obtained using the triaxial and
direct shear apparatuses. Skempton (1964) stated that the same
effective stress shear strength parameters were obtained from
tests conducted on eight specimens of Boulder Clay using the
direct shear and triaxial devices. Casagrande and Poulos (1964)
presented the results of CD direct shear and CD triaxial tests
performed on compacted specimens of a lean clay which
showed that about the same shear strength envelope was
obtained with the two tests. Moon (1984) performed CD direct
shear and CU triaxial tests on undisturbed samples of a fat clay
and found differences of less than one degree for the effective
stress friction angle and less than 8 kPa for the effective
cohesion intercept obtained from these two tests using the
maximum principal stress ratio as the failure criterion. Thomson
and Kjartanson (1985) performed CD direct shear tests and CD
and CU triaxial tests on undisturbed samples of a lean clay and
a fat clay and found that the results plotted on the same failure
envelope. Abdel-Ghaffar (1990) compiled results from the
literature where direct shear and triaxial tests were performed
on undisturbed samples of the same soil. He concluded that the
direct shear and triaxial devices provide comparable values for
the effective stress friction angle and cohesion intercept.
Maccarini (1993) performed CD direct shear and CD triaxial
tests on a residual soil from Rio de Janeiro. For these tests, the
tests specimens were oriented so that the failure plane in both
devices coincided with the direction of stratification of the soil.
The stratification of the soil had a dip angle of 25°. Based on the
results obtained, Maccarini concluded that similar values of
effective stress cohesion and friction angle are obtained from
both tests if the stratification is taken into account.
Based on the results presented above, it can be seen that the
available information in the literature is divided on whether
CD Direct Shear
CU Triaxial
35
30
25
20
15
10
0
20
40
60
80
100
120
Plasticity Index (%)
Figure 1. Relationship between effective stress friction angle and
plasticity index for CD direct shear tests and CU triaxial tests on
undisturbed samples.
3.2
Remolded test specimens
Three remolded clays were tested to allow a comparison of the
fully softened shear strength measured in CD direct shear tests
and CU triaxial tests. The index properties of these clays are
shown are in Table 1. NOVA clay was obtained from a site in
Northern Virginia in Fairfax County. Vicksburg Buckshot Clay
(VBC) was obtained from the stockpile maintained at the
Engineering Research and Development Center of the U.S.
Army Corps of Engineers. It has been the subject of many
research projects (Ladd and Preston 1965; Mitchell et al. 1965;
Al-Hussaini and Townsend 1974). Colorado Clay is a lean clay
(CL) from Silverthorne, Colorado.
To prepare the remolded samples, the soils were first soaked
in water for at least 48 hours, and then processed through a No.
Technical Committee 101 - Session II / Comité technique 101 - Session II
40 (425 µm) sieve. At this point, the samples were at a water
content much greater than the liquid limit and were therefore
air-dried to reduce the water content. The soil was considered to
be at the desired water content when 23 to 25 blows using a
Casagrande liquid limit cup were required to close the groove
cut in the soil as specified in ASTM D4318-10.
preferential structure. Upon visual inspection, the New Orleans
soils appear to be relatively homogenous. Distinct layering is
often not discernible. Shown on the left side of Figure 5 is a
section cut through an undisturbed New Orleans sample. The
right side of Figure 5 shows the same sample after drying. The
horizontal layers, which can represent planes of weakness,
become evident after drying.
Table 1. Soil index properties.
Soil
Specific
Gravity
LL
PL
PI
Clay-sized
Fraction
Colorado Clay
2.78
42
22
20
23.7
NOVA Clay
2.80
66
28
38
16.8
VBC
2.79
78
26
52
68.9
Specimen
Container
Water Bath
To perform CD direct shear tests, the test specimens were
formed directly in the direct shear box using a spatula or
“piped” from a pastry bag. Care was taken to form the test
specimens without entrapping air-bubbles. Prior to shearing, the
samples were consolidated under Ko conditions to vertical
pressures ranging from 24 kPa to 288 kPa. The samples were
then sheared slow enough to allow full dissipation of the pore
pressures generated during shear using the criterion presented in
ASTM D3080-11.
Triaxial test specimens were more difficult to form for tests
at low effective consolidation stresses. At a water content equal
to the liquid limit, the soil does not have the shear strength
required to be formed into triaxial test specimens that can be
installed in a triaxial cell. For this reason, a batch of soil was
first consolidated in a batch consolidometer under Ko conditions
to a vertical pressure of 38 kPa. The batch consolidometer used
in this investigation is shown in Figure 2. After the batch of soil
reached the end of primary consolidation, the sample was
extruded from the specimen container, and triaxial test
specimens were trimmed. The batch consolidometer formed
samples that had a diameter of 15 cm, which allowed five to six
3.6 cm diameter test specimens to be trimmed. The triaxial test
specimens were then installed in a triaxial cell, back pressure
saturated, and consolidated to an all-around pressure ranging
from 48 kPa to 483 kPa prior to shearing. The samples were
sheared slow enough to allow equilibration of the pore pressures
according to the criterion presented by Head (1986).
The results from the triaxial and direct shear tests performed
on remolded samples are presented in Figure 3. The envelopes
were clearly non-linear, and a power function was fit to the test
results. The power function used was consistent with the format
described by Lade (2010) (see Eq. 1).
 ' 
  a  pa  
 pa 
with:
b
(1)
′
= normal effective stress on the failure plane
= atmospheric pressure in the same units as ′.
pa
a and b = curve fit parameters.
The results show that no significant differences were
obtained in the failure envelope determined with the direct shear
device and triaxial device for these remolded samples.
4
DISCUSSION
The difference in the results of the triaxial and direct shear tests
on undisturbed and remolded samples can be explained, in part,
by the difference in soil structure. The undisturbed tests
specimens obtained from the greater New Orleans area were
lacustrine and riverine alluvial deposits. These soils were
deposited in horizontal layers, and could be expected to have a
319
Pneumatic
Piston
Figure 2. Batch consolidometer.
This horizontal layering can influence the shear strength
measured using the direct shear device. In the direct shear
device, the failure plane is horizontal. This forced failure plane
can sometimes coincide with the natural layering or planes of
weakness. In the triaxial device, the failure plane is not
predetermined by the configuration of the device and horizontal
planes of weakness would not be expected to control the
measured shear strength. The difference in shear strength as a
function of failure plane orientation has been documented by
many investigators (Duncan and Seed 1966a; b; Filz et al.
1992).
Remolding soil destroys any previous layering or structure
that might have been present, and the resulting sample is more
or less homogenous. The influence of the failure plane
orientation should be much less prevalent as compared to
undisturbed samples. This suggests that fully softened shear
strength (i.e. remolded normally consolidated peak shear
strength) might be less dependent on the choice of the test
apparatus than undisturbed shear strength.
5
CONCLUSIONS
The choice of using direct shear tests or triaxial tests to
determine drained shear strength parameters can be important
for natural soil deposits. Although these two different test
methods can often provide similar results in some soil deposits,
there is considerable evidence that direct shear tests provide
much lower friction angles in riverine and lacustrine alluvial
deposits than triaxial tests. A comparison of numerous CD
direct shear and CU triaxial test results conducted on alluvial
soils from the greater New Orleans area show that the friction
angle determined from the direct shear apparatus is normally
about 2 to 5 degrees lower than that determined using the
triaxial apparatus. This can be attributed to the anisotropic
shear strength characteristics of the alluvial soils.
The difference in the results of the two test devices is much
less when remolded test specimens are used. The remolding
process destroys the anisotropic fabric, and the shear strength
parameters are not as dependent on the orientation of the failure
plane. When fully softened shear strength parameters are
desired, direct shear and triaxial test apparatuses appear to
provide comparable results.
Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
ASTM Standard D3080. (2011). "Standard test method for direct shear
test of soil under consolidated drained conditions." ASTM
International.
West
Conshohocken,
PA.
2011.
DOI:
10.1520/D3080_D3080M-11. www.astm.org.
ASTM Standard D4318. (2010). "Standard test method for liquid limit,
plastic limit, and plasticity index of soils." ASTM International.
West Conshohocken, PA. 2010. 1 10.1520/D4318-10.
www.astm.org.
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investigation of stress-deformation and strength characteristics
of compacted clays.” Harvard Soil Mechanics Series No. 74.
Dunbar, J. B., and Britsch III, L. D. (2008). “Geology of the New
Orleans area and the canal levee failures.” Journal of
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582.
Duncan, J. M., and Seed, H. B. (1966a). “Anisotropy and stress
reorientation in clay.” Journal of the Soil Mechanics and
Foundations Division, 92(5), 21–50.
Duncan, J. M., and Seed, H. B. (1966b). “Strength variation along
failure surfaces in clay.” Journal of the Soil Mechanics and
Foundations Division, 92(6), 81–104.
Filz, G. M., Brandon, T. L., and Duncan, J. M. (1992). “Back analysis
of Olmsted Landslide Using Anistropic Strengths.”
Transportation Research Record 1343, Transportation Research
Board, National Research Council, National Academy Press,
Washington, DC, 72–78.
Head, K. H. (1986). Manual of Soil Laboratory Testing. Vol. 3:
Effective Stress Tests. John Wiley & Sons, 1238.
Hvorslev, M. J. (1960). “Physical components of the shear strength of
saturated clays.” Research Conference on Shear Strength of
Cohesive Soils, Boulder, Colorado, 169–273.
Ladd, C. C., and Preston, W. B. (1965). On the Secondary Compression
of Saturated Clays. Vicksburg, MS, 116.
Lade, P. V. (2010). “The mechanics of surficial failure in soil slopes.”
Engineering Geology, 114(1-2), 57–64.
Maccarini, M. (1993). “A comparison of direct shear box tests with
triaxial compression tests for a residual soil.” Geotechnical and
Geological Engineering, 11(2), 69–80.
Mitchell, J. K., Shen, C., and Monismith, C. L. (1965). Behavior of
Stabilized Soils Under Repeated Loading. Report I. Background,
Equipment, Preliminary Investigations, Repeated Compression
and Flexure Tests on Cement-Treated Silty Clay. Vicksburg,
MS, 136.
Moon, A. T. (1984). “Effective shear strength parameters for stiff
fissured clays.” 4th Australia-New Zealand Conference on
Geomechanics, 107–111.
Potts, D. M., Dounias, G. T., and Vaughan, P. R. (1987). “Finite
element analysis of the direct shear box test.” Géotechnique,
37(1), 11–23.
Saada, A. S., and Townsend, F. C. (1981). “State of the art: laboratory
strength testing of soils.” Laboratory shear strength of soil.
ASTM STP 740, R. N. Yong and F. C. Towsend, eds., American
Society for Testing and Materials, 7–77.
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Géotechnique, 14(2), 77–102.
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Géotechnique, 20(3), 320–324.
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a cut slope in stiff clay.” Canadian Geotechnical Journal, 22(2),
286–297.
Colorado Clay
Shear Stress (kPa)
200
CU Triaxial
a = 0.601
b = 0.924
150
CD Direct Shear
a = 0.646
b = 0.905
100
50
CD Direct Shear
CU Triaxial
0
0
100
200
300
400
Effective Normal Stress (kPa)
NOVA Clay
Shear Stress (kPa)
200
CU Triaxial
a = 0.533
b = 0.838
150
CD Direct Shear
a = 0.528
b = 0.783
100
50
CD Direct Shear
CU Triaxial
0
0
100
200
300
400
Effective Normal Stress (kPa)
VBC
Shear Stress (kPa)
200
CU Triaxial
a = 0.461
b = 0.767
150
CD Direct Shear
a = 0.427
b = 0.729
100
50
0
CD Direct Shear
CU Triaxial
0
100
200
300
400
Effective Normal Stress (kPa)
Figure 3. CD direct shear and CU triaxial test results on remolded test
specimens.
Figure 4. Undisturbed soil specimen before and after drying.
6
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Significance of the Cohesion Intercept in Soil Mechanics.”
Thesis presented to University of Illinois, Urbana-Champaign,
IL in partial fulfillment of the requirements for the degree of
Doctor of Philosophy, 262.
Al-Hussaini, M. M., and Townsend, F. C. (1974). Investigation of
Tensile Testing of Compacted Soils. Vicksburg, MS, 76.
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